Endovascular advances in aneurysm repair

Leader of the University College London Hospital (UCLH) Multidisciplinary Endovascular Team, Prof. Harris has observed that the major focus of vascular surgery has been on high-risk diseases with high mortality, associated with the proximal aorta. In the case of abdominal aortic aneurysms, the techniques are now well established, with the trend being for endovascular treatment to replace open surgery. “Randomised trials have shown that there is a three-fold less risk in treating abdominal aortic aneuryms through minimally invasive endovascular approaches compared to open surgery. This is because it is minimally invasive and is less traumatic for the patient,” said Prof. Harris. “The expectation is that if we can treat diseases of the aorta that are closer to the heart using endovascular approaches, similar improvements in outcomes could be achieved. However, there are challenges – the closer the proximity to the heart, the bigger the aorta is. There is also less uniformity of diameter and greater forces placed on the endograft or device used. In addition, there is also a temporary obstruction to the blood flow involved with the procedure, which has the greatest impact the closer this is performed to the heart. “At UCLH we are focusing on the application of established endovascular treatments to the more proximal aorta, which is the final frontier, as far as endovascular interventions are concerned. Where the whole of the aorta is diseased, we are looking at whether it is possible to realign and replace the whole of the aorta with endovascular methods including the valve,” he continued. He explained that the valve can be treated with a transcatheter implant and it is possible to treat separate parts of the aorta with stent grafts, but the goal is to be able to use endovascular approaches to treat more widespread disease such as in patients with Marfan syndrome. “Although, open surgery is still the standard of care for the treatment of the arch of aorta and the ascending aorta, it is now widely accepted that endovascular methods are the treatments of choice for the abdominal aorta and descending thoracic aorta. These are the parts of the aorta that are relatively straight. “However, to replace the arch of the aorta using open surgery, you need a period of circulatory arrest. Usually this is performed using suspended animation of the patient and brain cooling, but it is very traumatic for the patient and excludes a large number of elderly patients and those with serious comorbidities. Therefore, the aim is to find a minimally invasive treatment that does not involve open heart surgery, cardiopulmonary bypass, profound hypothermia and circulatory arrest. “We are very close to achieving this. There are experimental devices that are currently being trialled, which will enable us to treat the arch and the ascending aorta,” he revealed, adding that UCLH is evaluating various approaches – including in situ fenestration. “In situ fenstration involves inserting an endograft across a branch of the left subclavian artery. Once situated in the artery, a perforation can be created in this endograft and then a stent is inserted. “This allows for the use of an off-theshelf solution, which is important – as patients usually present as emergency cases and there is no time to create a tailor made device,” Prof. Harris explained. “The problem when treating the arch or the aorta is that there is a period of time when you over stent and stop the blood flow to all of the blood vessels supplying the brain. You can perform the in situ fenestration quite quickly, in stages, or combine it with bypasses in the neck. It is possible to overcome some of the problems, but it is not straightforward,” he continued. “Our approach has been to use a heart/lung machine to perfuse the head to ensure maximum patient safety. This is still preferable to open surgery as we do not have to open the chest, cardiac arrest is not required and we do not have to cool the patient, although this approach is still not completely non-invasive.”

Ready-to-use fenestrated graft

He acknowledged that there are questions over the durability of this approach in the long term, as there are concerns that movement between the stent and hole in the fabric may lead to a leak or propagation of tears in the fabric. However, UCLH has responded to this risk by asking a manufacturer to develop a ready-to-fenestrate stent graft with a modified fabric that will allow the surgeon to fenestrate in situ, in such a way that a sealing ring is formed and tears are avoided. “We believe that this is could be made possible by integrating threads of nytinol into the fabric. The nytinol fibres condense around the perforation to perform a sealing ring to ensure a more durable seal. This has not been completely perfected just yet, but we believe this could be a possible solution.” The concept of creating a ready-to-use fenestrated graft is also being extended to the thoraco-abdominal aorta i.e. the abdominal visceral branches. The hypothetical stent-graft would feature ready-made holes (fenestrations) for perfusion of the renal arteries and the mesenteric arteries. These holes need to be positioned very accurately as they have to match the patient’s anatomy exactly to avoid blocking the artery and curtailing blood flow. “Although this is a commonly used approach, the process of planning, design and construction of the customised fenestrated stent graft can take at least six weeks, which exposes the patient to risk. It is also very expensive as it is labour intensive,” Prof. Harris continued. As there is a limited range of positions for the visceral arteries in relation to the aorta, he said that it is possible to manufacture a fenestrated stent graft that is ready to use with pre-loaded wires for controlled placement of multiple fenestrations. Instead of having to engage in detailed planning of the operation in advance, via a CT workstation, then having to send off the anatomic measurements to a manufacturer, the off-the-shelf graft could be modified and deployed inside the patient to select the appropriate fenetrations to match that patient’s anatomy. “Not only are there time and cost advantages to this approach, but the major advantage is that in the case of customised fenestrated stent grafts, the patient’s anatomy may change when it comes to the operation and a degree of adjustment is therefore needed,” he added. “There is room for error in this process. But by using sophisticated 3D imaging, coupled with the new approach, the pre-loaded wires can be matched to suit the appropriate position required as dictated by the patient’s anatomy. The wires which are not required are simply removed and those selected may be used to match the fenestration in the fabric to the vessel.”